The present invention may be used in a tri-level printing system that utilizes a pulse width modulated raster output scanner (ROS) to produce a tri-level latent image such as the Xerox 4850 Highlight Color Laser Printing System. The tri-level latent image produced in such a system is developed and transferred to an output sheet or similar print medium. In tri-level or highlight color imaging, unlike conventional xerography, upon exposure, three charge levels are produced on the charge-retentive surface. The highly charged (i.e. unexposed) areas are developed with toner, and the area most fully discharged is also developed, but with a toner of a different color, referred to as the highlight color. The charge retentive surface is exposed at three levels; zero exposure, intermediate exposure, and full exposure, which correspond to three charge levels. The three resulting levels can be developed to print, for example, black, white, and a single color.
As described in U.S. patent application Ser. No. 07/756,643, the relevant portions of which are hereby incorporated by reference, charged portions of a photoreceptor surface are advanced through an exposure station. Briefly, at the exposure station the photoreceptor surface is exposed by a tri-level ROS unit which causes the surface to be discharged in accordance with the output from an image source. This scan produces three separate discharge regions on the photoreceptor, each region exposed at one of three possible levels: (1) zero exposure which results in a voltage equal to the dark-decay potential and will be developed using charged area development (CAD); (2) full exposure, which results in a low voltage level and is developed using discharged area development (DAD); and (3) intermediate exposure, which yields an intermediate voltage level that does not result in development by CAD or DAD, yielding a background region on the print. Subsequent to development, the developed image is transferred and fused to the print medium using techniques commonly known for tri-level xerographic printing systems.
Heretofore, various methods and apparatus have been used to control and enhance the output of ROS based printing systems. Moreover, several scanning techniques are known to obtain tri-level exposure imaging. To obtain higher spatial resolution, a pulsed imaging scanner can be utilized. This pulsed imaging scanner is also referred to as a Scophony scanner in an article in Optical Engineering, Vol. 24, No. 1, Jan./Feb. 1985, Scophony Spatial Light Modulator, by Richard Johnson et al., whose contents are hereby incorporated by reference. The following disclosures may be relevant:
U.S. Pat. No. 4,347,523, also to Ohara, discloses an apparatus of general interest which uses an input signal to address pulse numbers with corresponding pulse width selection numbers.
U.S. Pat. No. 4,375,065 to Ohara describes an apparatus of general interest that uses pulse number and pulse position modulation to control a laser beam.
U.S. Pat. No. 4,390,882 to Ohara et al. discloses for an image processing apparatus a method of adjusting the image density by controlling the on time of the laser. Control of the laser on time is performed by a multivibrator having a variable RC time constant.
U.S. Pat. No. 4,437,122 to Walsh et al. teaches an improved method of converting low resolution images into images of higher resolution for printing so as to simultaneously increase density and smooth character edges. In a CRT display or hardcopy output apparatus, the invention is accomplished by converting an original pixel into a higher resolution 3.times.3 enhanced representation. The status of each of the nine elements in the enhanced representation is determined as a result of an examination of the neighboring pixels of the original pixel.
U.S. Pat. No. 4,544,264 and U.S. Pat. No. 4,625,222, both issued to Bassetti et al. describe enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scanlines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. No. 4,544,922 to Watanabe et al. teaches a smoothing circuit for an orthogonal matrix display. The circuit adds or removes a "small dot" on the display from either the first or last third of a dot clock (DCK) period which is one-third the period in which a standard dot of the original pattern is displayed.
U.S. Pat. No. 4,626,923 to Yoshida teaches an image processing apparatus for producing a halftone image in which the on time of the laser is controlled by both the image input data and a pulse width modulation circuit. The image data is transferred under control of clock signal, CLK. The pulse width modulation circuit includes a clock, CLKH, having a frequency three times that of CLK, which is used together with latches and AND gates to provide synchronous sub-pixel addressing.
U.S. Pat. No. 4,661,859 to Mailloux et al. describes an image processing circuit for producing a greyscale image in which the on time of the laser is controlled by both the video input data and the pulse width modulation circuit. The pulse width modulation circuit includes a clock having a frequency greater than the video data rate, which allows synchronous sub-pixel addressing.
U.S. Pat. No. 4,847,641 and U.S. Pat. No. 5,005,139 to Tung disclose print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap which was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 4,905,023 to Suzuki, describes an image forming apparatus using a plurality of conversion tables addressed by an input video image signal to generate pulses.
U.S. Pat. No. 4,926,268 to Kawamura et al. discloses an image processing apparatus which employs analog circuitry to produce a pulse-width modulated (PWM) output from a multi-level digital signal. As described, each analog signal is generated in synchronism with the pixel clocks.
U.S. Pat. No. 4,933,689 to Yoknis describes a method for enhancing a displayed image in a laser exposed dot matrix format to produce softened edge contours. Using three pulses, a central pulse plus leading and trailing enhancement pulses which are separated therefrom. The purpose of the leading and trailing pulses is to create a blurred or grayed region at the leading and trailing edges of each associated character.
U.S. Pat. No. 4,965,672 to Duke et al. discloses an apparatus for varying the width and position of pulses used to control a laser beam.
U.S. Pat. No. 5,041,848 to Gilbert et al. teaches a non-gray scale anti-aliasing method for smoothing the horizontal components of the edges of an image to be printed by a laser printer having unequal pixel resolutions in the horizontal and vertical dimensions. Working from an ideal outline of the image, the method smooths the edges of the digital outline by selectively modifying the on and off states of pixels on either side of the vertical transition point along the horizontal components at the edges of transition to produce a modified pixel representation.
U.S. Pat. No. 5,134,495 to Frazier et al. discloses a laser based imaging system which employs a resolution transformation method. The method uses the selective activation in overlapping areas between rasters (scan lines) . In one embodiment, a single interleaved pixel, between two scan lines, is formed by the sum of up to six laser pulses at pixel points on adjacent scanlines. In some cases the laser pulses are of insufficient intensity to produce a dot or mark at the point on the scanline where the center of the pulse is received.
U.S. Pat. No. 5,144,337 to Imamura et al. teaches an image forming apparatus suitable for changing the size of an output dot in a main and subscanning direction. Dot size and shape are controlled by pulse width modulation and power modulation applied to a laser diode.
U.S. Pat. No. 5,144,338 to Sakano discloses an image recorder which employs a pulse width modulated laser beam to control the recording position on a photoconductive drum. The position (left aligned, centered, or right aligned) and duration (12 ns, 20 ns, 32 ns, or 56 ns) of the pulse within a pixel interval is determined based upon the tone level of the pixel of interest and its relation to the tone levels of both preceding and following pixels.
U.S. Pat. No. 5,184,226 to Cianciosi describes a digital system for generating pulses from a series of data words, the relevant portions of which are hereby incorporated by reference. The system employs multiple RAM look-up tables for translating the data words into a series of corresponding pulses utilizing two channels to achieve the desired throughput.
U.S. Pat. No. 5,193,008 to Frazier et al. further describes the resolution enhancement apparatus as one which includes the ability to rasterize the image to be printed at twice the resolution of the printer. The printer then outputs the higher resolution image using an interleaving technique which generates developable dots between scan lines by energizing corresponding dots on adjacent scanlines at a level which will not be developed, but where the overlapping portion of the two corresponding dots will be developable.
U.S. Pat. No. 5,193,011 to Dir et al. discloses a system for printing gray levels without the need of a halftone cell. The system determines the pulse width for each pixel as a function of the gray level of the pixel, based upon an iterative comparison to an incrementing grey level clock. In one embodiment, a page-wide liquid crystal shutter is used to regulate the exposure of a photoconductive drum. The shutter may be toggled on and off multiple times for each pixel during the recording of a single row of the image.
U.S. Pat. No. 5,223,857 to Loce et al. describes a pulsed imaging ROS which utilizes pulse width modulation in conjunction with spatial filtering to form three exposure levels on a photoresponsive surface, each level being associated with a specific development color.
EP-A-361,538 by Goertzel et al. discloses a system for producing halftone images with sharpness or edge enhancement. The edge enhancement is achieved by employing a high resolution output device to print out halftone cells resulting in lower resolution "grey" blocks in stead of just black or white output.
L. Steidel in "Technology Overview: Resolution Enhancement Technologies for Laser Printers", LaserMaster Corp., discusses three currently available implementations for vertical resolution enhancement, Resolution Enhancement Technology, Paired Scan Line Scheme, and TurboRes. In all cases, the horizontal resolution of the laser scanner is increased by increasing the clock speed. On the other hand, the vertical resolution is enhanced by combining the weaker laser laser energy from a brief laser flash, which leaves only residual or fringe energy on the image drum at the periphery of a pixel of an adjacent pixel on a second scan line.
A preferred technique, capable of higher spatial resolution, is to use similar optical elements as the flying spot scanner (rotating polygon, laser light source, pre-polygon and post-polygon optics), but with an acoustooptic (A/O) modulator which illuminates many pixels at a given time, resulting in a scanner with a coherent imaging response. With this type of scan system, the exposure level, or levels at the image surface, can be controlled by controlling the drive level of the A/O modulator dependent on the video data. In a tri-level system, two drive levels are used, one for the white exposure and a second higher drive level for the DAD, or highlight color, exposure.
Instead of obtaining an intermediate exposure level by controlling the acoustic amplitude, an intermediate exposure may be produced using pulse width modulation in a pulsed imaging system in conjunction with spatial filtering. Using an intuitive, or conventional approach to pulsed width modulation, in which the pulses are centered on the pixels, not only leads to color text and graphics in output prints that may have a "bloated" or bolded appearance, but also reproduces what are commonly referred to as "jaggies" when digitized images are printed. Most often, the jaggies are visible along the edges of angled or curved lines, and along characters produced with rasterized fonts. According to one aspect of the present invention, and in a preferred embodiment, the jaggies problem is eliminated for CAD developed regions (i.e., angled or curved black edges) by trimming the nearest edge of white video pulses, used to produce neighboring pixels, to produce a "grey" output.
In accordance with the present invention, there is provided a method for enhancing charged area developed regions on a photoresponsive member in a pulsed imaging, pulse width modulated printing system used for creating tri-level images on a photoresponsive member. The method comprises the steps of: converting a series of video data into a series of composite analog video pulses corresponding to a plurality of pixel periods, each pixel period having a composite video pulse representing a charged area, discharged area, and intermediate discharged area to be formed on the surface of said photoreceptor member, said composite video pulses being applied to an acoustooptic modulator to modulate a laser beam used to expose regions of the photoresponsive member; identifying, a subset of the video data to be altered so as to improve the appearance of the charged area developed regions of the output image; and altering selected video data within the subset by modifying a video pulse corresponding to the selected video data.
In accordance with another aspect of the present invention, there is provided a charged area development enhancement apparatus for a pulsed imaging, pulse width modulated printing system which creates tri-level images on a photoresponsive member, including: an acoustooptic modulator to modulate a laser beam used to expose regions of the photoresponsive member; means for converting a stream of video data, representing a plurality of pixels, into composite analog video pulses having uniform pixel periods, each pixel period defining a composite video pulse representing a charged area, discharged area, and intermediate discharged area to be formed on the surface of said photoreceptor member, said composite video pulses being applied to the acoustooptic modulator; means, responsive to the stream of video data, for identifying a subset of video data therein to be altered so as to improve the appearance of the charged area developed regions of the output image; and means for altering the video data representing a selected pixel within the subset by modifying the video pulse corresponding to the selected pixel.